Zoom lenses and aperture question

D

DW_

Senior Member
I have a question about zoom lenses and aperture and I'm hoping there's an optical physicist in the room that can help me. There are 2 distinct families of lenses, one with constant aperture thru the various focal lengths and the other with an increasing (or decreasing, not sure which adjective is the right one) aperture as you increase the focal length.

So my question is this - why?

Take for instance a 70-300mm F/ 4-5.6 Why is it that when you increase the focal length you must decrease the maximum aperture? Clearly the larger aperture is mechanically available within the lens itself. Is it because the beam of light has narrowed as the focal length has increases such that the light itself is no larger than 5.6 so by virtue of the light rays themselves you cannot have a larger aperture? Then compare this with a constant aperture lens such as the 70-200mm F 2.8, how is it that at full zoom you can still have access to a 2.8 f-stop? From a mechanical standpoint, it seems like it would be easier to make a lens who's aperture is not a function of the focal length, doesn't it? But yet the constant aperture lenses are much more expensive which implies they're more demanding to manufacture.

Hopefully someone out there in internet-land will understand my rambling question and have a good answer for me.

TIA
 
D

DW_

Senior Member
Great link but it still doesn't answer my question on why the max aperture varies on some lenses but doesn't on others. I'm thinking it's not an easy answer
 
D

Don Kuykendall_RIP

RIP :(
[h=1]Nikon AF-S Nikkor 70-200mm f/2.8G ED VR II Lens Price: $2,396.95 [/h][h=1][/h][h=1]Nikon 55-200mm f/4-5.6G ED IF AF-S DX VR Nikkor Zoom Lens Price: $249.00[/h]Nikon 70-300mm f/4.5-5.6G ED IF AF-S VR Nikkor Zoom Lens Price: $589.00

One of the biggest reasons in price. The firstr two are more apples to apples. Both just go to 200mm but there is more than $2,000 difference. With the cost there is much larger glass and weight.
 
F

fotojack

Senior Member
It's actually much more expensive to manufacture a constant aperture lens than it is to manufacture a variable aperture lens. There's a lot of physics and geometry involved in the making of a constant aperture lens.....hence the greater cost. I think for your question to be answered, you would need to ask a lens maker. It's far above my knowledge in how they work out the actual process, but I'm sure there must be a definitive answer. I'd be curious to know, too, from an actual manufacturing standpoint. :)
 
D

DW_

Senior Member
fotojack said:
It's actually much more expensive to manufacture a constant aperture lens than it is to manufacture a variable aperture lens. There's a lot of physics and geometry involved in the making of a constant aperture lens.....hence the greater cost. I think for your question to be answered, you would need to ask a lens maker. It's far above my knowledge in how they work out the actual process, but I'm sure there must be a definitive answer. I'd be curious to know, too, from an actual manufacturing standpoint. :)
Click to expand...

Yeah, that's kinda on the lines that I was thinking. My guess is the cheaper, more traditional lenses, suffer from a smaller aperture due to the stream of light narrowing as the focal length increases, which would not be a function of mechanics but rather a function of optics. Whereas the constant aperture lenses must do something unique with the stream of light that keeps it from narrowing as the focal length increases. But again, this is just my ponderings and I may be way off base with them. I think it would be interesting to know exactly how a constant aperture lens works and why all lenses are not that way.
 
C

Carolina Photo Guy

Senior Member
DWPhotos said:
Great link but it still doesn't answer my question on why the max aperture varies on some lenses but doesn't on others. I'm thinking it's not an easy answer
Click to expand...

I pointed you to this link so that you would be able to start figuring out exactly what you really wanted to know. I took the question to be akin to "Why does Government Motors make a Corvette and Ford does not?"

Sometimes, the road to an answer is circuitous, but never boring.

Sorry if it didn't help. :(
 
E

Eye-level

Banned
OK I will give it a shot...in a nutshell fixed aperture lenses are way more sophisticated, typically use a "more" special ground glass, and the elements tend to be much larger than the ones used in variable aperture lenses (hence their greater size and weight), and finally they typically contain "special" types of elements. On many modern zoom lens mounted to digital cameras you are able to fix the aperture as long as it stays within the range of the lens...eg you have a 4-5.6 lens you can find an option in one of your camera menus and set the fixed aperture at F4 you can then zoom the lens in or out and it will stay at F4 the trade off is that you lose optical quality the further out you zoom where with the variable aperture it is optimized to perform how it performs at the various zoom ranges according to it's lens formula. (A high dollar fixed aperture lens typically has so called floating elements which change position as the lens is zoomed in or out thus maintaining the light at the sensor at a constant strength. Floating elements are much harder to manufacture and thus way more expensive. Edit they are not really floating elements but they are special and there also tend to be more of them in fixed aperture lenses)

So basically what that means is that it is not so much the physics of the lens per se (athough that is of utmost concern to the lens designer) but has more to do with cheaper and faster manufacturing process.

Now let's look into the very near future...lens grinding technology and computer aided lens formula design is progressing by leaps and bounds so what will probably happen is they will be able to make these variable aperture lenses perform better and better until they are just as good as a fixed aperture lens. Leica and Nikon and Canon are already making very very good variable aperture zooms and one day we can probably expect all zoom lenses to be made this way.

It is a very similiar situation with regard to fixed focal length lenses...the new formulas and new coatings are making for better lenses...

Everything is computers now...think about how much more efficient modern day cars are...it is because the engines are computer designed for optimum performance but they still burn the same old gas.
 
Last edited:
R

Rick M

Senior Member
My Guess-

As the optics of the zoom move it chokes down on the light rays, forcing a smaller (higher) aperature at the back end.

The constant aperature lenses have larger forward optics (hence the higher cost). The larger diameter lens does not choke down the light as much, allowing for a larger (constant) aperature at the back end of the lens.
 
Last edited:
Top